1. Field of the Invention
The present invention relates to a nuclear magnetic resonance spectrometer and also to a method of magnetic field correction.
2. Description of Related Art
In NMR (nuclear magnetic resonance) spectroscopy, the resonance frequency ω is in proportion to the strength B0 of the static magnetic field and to the gyromagnetic ratio γ of nuclear spins, as given byωideal=γB0 
Therefore, the positions of peaks appearing in an NMR spectrum directly reflect the strength of the static magnetic field. As a result, the distribution (lineshape) of peak positions reflects the distribution of the static magnetic field. Accordingly, in high-resolution NMR spectroscopy, there is an intrinsic requirement for high static field homogeneity. Where static magnetic field strengths are distributed spatially, if the difference with an ideal static magnetic field strength is given by δB(r), then the distribution of resonant frequencies is given byωunshimmed(r)=γ(B0+δB(r))
A method of correcting a static magnetic field by electrically energizing electromagnets (known as a shim coil set) to produce a corrective magnetic field so as to correct the static magnetic field is widely accepted to achieve a uniform static magnetic field (see, for example, JP-A-8-316031). Let δBshim(r) be the strength of a magnetic field produced by a shim coil set. The resulting static magnetic field is given byωshimmed(r)=γ(B0+δB(r)+δBshim(r))
Accordingly, if the shim coil set is energized with an electrical current satisfying the relationship, δB(r)+δBshim(r)=0, then an ideal homogeneous magnetic field is obtained. Generally, numerous shim coils are mounted in an NMR instrument to produce various corrective magnetic fields. It is important to determine what magnitude of electrical current should be fed to what shim coil in accomplishing a homogeneous static magnetic field.
A shim coil generally used in an NMR instrument is so placed that its Z-axis lies parallel to the static magnetic field and that the strength of the produced corrective magnetic field varies according to a spherical harmonic function. This permits various terms of the corrective magnetic field components to be adjusted independently. Consequently, efficient shim adjustments can be accomplished. In particular, a corrective magnetic field applied by a shim coil in the axial direction (Z-axis direction) can be given by Eq. (1). Corrective magnetic fields applied by shim coils in radial directions (X-axis and Y-axis directions) can be given by the following Eqs. (2) and (3)z:Bz(n)∝bz(n)rnPn(cos θ)  (1)Bm(n)e∝bm(n)rnPnmn(cos θ)cos mφ  (2)Bm(n)o∝bm(n)rnPnmn(cos θ)sin mφ  (3)where
n is an integer equal to or greater than unity,
m is an integer satisfying the relationship |m|≦n (when n=1, m=−1, 0, 1; when n=2, m=−2, −1, 0, 1, 2),
z is an axis extending along the static magnetic field,
bz(n) corresponds to the magnitude of the current flowing through the shim coil,
r, θ, φ are coordinate-axis components of polar coordinates, and
Pnmn(cos θ) are associated Legendre functions.
If the sample is spun about the Z-axis, inhomogeneities of the magnetic field along the Z-axis (i.e., within the XY-plane) are averaged. Inhomogeneities about the Z-axis appear only as spinning sidebands. Therefore, if the spinning sidebands are neglected, field inhomogeneities can be corrected only with the terms Bz(n) regarding the Z-axis direction. Where the sample is not spun, it is necessary to adjust all the terms Bz(n), Bm(n)e, and Bm(n)e, and Bm(n)o.
In NMR spectroscopy, magic angle spinning (MAS) has enjoyed wide acceptance. That is, a sample is spun about an axis tilted at magic angle θm from a static magnetic field. Especially, in solid-state NMR spectroscopy, MAS NMR is used as a general technique. The magic angle θm is given by
      θ    m    =            cos              -        1              ⁡          (              1                  3                    )      
At this time, inhomogeneities in the magnetic field around the axis of rotation of the sample are averaged out by MAS. Usually, in MAS NMR, spinning sidebands due to field inhomogeneities present no problems because the spinning rate of the sample is sufficiently larger than field inhomogeneities along the axis of spinning of the sample. Consequently, if the field inhomogeneities in the direction of spinning of the sample are corrected, high-resolution NMR spectra are accomplished.
A method of correcting a static magnetic field, which is applied to a sample to be investigated by NMR, by the use of shim coils is now described.
In an NMR spectrum, peaks appear at various unshimmed positions (ωunshimmed(r)). If the static magnetic field within the sample is uniform, signals appear only in a very narrow range of frequencies. Also, the peak intensities are maximized. Accordingly, the current flowing through each shim coil is so adjusted that the intensity of a peak appearing in an NMR spectrum maximizes and that the linewidth minimizes. It is also possible to adjust the current value through the shim coil, using an NMR signal to be observed directly. Furthermore, the adjustment can be carried out using another NMR signal (e.g., in the case of solution NMR spectroscopy, a 2H NMR signal arising from a solvent). In cases of solid samples many of which show short transverse magnetization relaxation times, the current value of the shim coil is adjusted using a reference sample showing a long transverse magnetization relaxation time. Then, the sample is exchanged. Thereafter, an NMR measurement can be performed.
However, in the above-described method of adjusting the current in the shim coil while directly observing the NMR signal, signals appearing at various locations within a sample are summed up and detected as an NMR signal. That is, in this method, it is impossible to obtain information indicating what position within the sample is shifted by what frequency (i.e., the static magnetic field is shifted). Therefore, it is impossible to forecast what shim coil should be electrically energized in order to improve the homogeneity of the static magnetic field. Furthermore, in order to improve the homogeneity of the static magnetic field by this method, plural NMR measurements are performed, for example, while varying the current fed to the shim coil until the linewidth of the resulting NMR signal is observed to be reduced and the signal intensity is observed to be increased. In this method, however, plural NMR measurements are needed and so this method is time-consuming. Furthermore, there is another problem that different results arise when an operator having different skills manipulates the instrument. Additionally, the homogeneity is evaluated using only linewidth and intensity of an NMR signal. Consequently, the instrument's operator often falls into local solutions.
A method using gradient shimming is known as a method of correcting a static magnetic field. In the method of gradient shimming, gradient magnetic field pulses and an echo method are combined, so that the position of the sample and the strength of the static magnetic field in that position (static magnetic field map) can be detected.
In the gradient shimming method, an unshimmed static magnetic field map and static magnetic field maps obtained when shim coils are energized with electrical currents are measured. The unshimmed static magnetic field map indicates how the static magnetic field is distributed. The static magnetic field maps show how the magnetic field is distributed when what currents are fed to what shim coils. These measurements make it possible to calculate what magnitudes of currents should be fed to what shim coils to make uniform the magnetic field.
For example, in solution NMR spectroscopy, the direction of a static magnetic field is taken along the Z-axis. An elongated sample tube is so positioned that its longitudinal axis lies along the Z-axis. In solution NMR, the magnetic field distributions along the X- and Y-axes are averaged by neglecting the magnetic field distributions along the X- and Y-axes or spinning the sample about the Z-axis. Thus, the homogeneity of the static magnetic field is improved by adjusting only the shim terms Bz(n) (see Eq. (1) above) expanded about the Z-axis by the use of gradient shimming. This method is known as one-axis gradient shimming.
However, the situation is different in MAS NMR. In solid-state NMR employing MAS, the longitudinal direction of an elongated sample tube does not lie on the Z-axis. Rather, the sample is spun about an axis (Ztilt-axis) tilted at the angle θm from the Z-axis. The magnetic field distribution around the Ztilt-axis can be averaged by sample spinning. Accordingly, the magnetic field distribution along the Ztilt-axis may be corrected. However, the Z-axis and the Ztilt-axis are different. Therefore, efficient magnetic field correction cannot be achieved with shim terms expanded around the Z-axis. For example, the strength of the magnetic field along the Ztilt-axis does not vary, for example, whatever the field component Bz(2) is varied. Accordingly, in an MAS NMR instrument, the one-axis gradient shimming which is used in the aforementioned solution NMR spectroscopy and which varies the shim terms Bz(n) cannot be utilized in some cases.
In this way, where NMR measurements are performed while spinning the sample about an axis tilted from the direction of the static magnetic field (Z-axis direction), the static magnetic field cannot be efficiently corrected in some cases.
A completely three-dimensional magnetic field distribution map is obtained by using three-dimensional NMR imaging. Therefore, if a sample is spun about an axis tilted from the direction of the static magnetic field (in the direction of the Z-axis), the magnetic field can be corrected. For example, even in solid-state NMR spectroscopy employing MAS, the magnetic field can be corrected. However, it is necessary to obtain a three-dimensional magnetic field distribution. Hence, it is time-consuming to make measurements. Furthermore, there is the problem that the instrument is complicated because a three-dimensional gradient magnetic field pulse system is required.